The present invention relates to an internal sizing agent in the neutral range for paper, paperboard, cardboard and other cellulose containing materials, as well as to a single stage method for internal sizing with such an agent.
An important additive to sizing which has been used for a long time in the paper making industry is known to be aluminum sulfate, which causes the precipitation and fixation of the sizing component, e.g. rosin, on the cellulose fiber. However, the pH that develops due to the hydrolytic splitting, depends on the aluminum sulfate concentration and is between 4.2 and about 4.8, i.e. in the distinctly acid range. This may lead to corrosion of screens and machine parts. Moreover, paper manufactured in this manner has poor aging resistance and reduced mechanical strength. A particular drawback of the acid pH is encountered when papers are to be manufactured which are streaked or mixed with calcium carbonate, e.g. due to the inclusion of waste paper, since the decomposition of calcium carbonate begins, although weakly, in the vicinity of the neutral point i.e., pH 6.5. The development of carbonic acid as well as the calcium ions going into solution may lead to considerable disturbances in the paper manufacture and may worsen the sizing of the paper.
For that reason, there has been no lack of experiments with the aim of overcoming the drawbacks and difficulties observed in connection with paper sizing when aluminum sulfate is used as the flocculation and fixing agent. However, attempts involving a raising of the pH, perhaps by reduction of the quantity of aluminum sulfate and replacement of the "saved" aluminum sulfate by a cationic retention agent, have not demonstrated the desired success. The pH of the fiber suspension could be raised in this way only to values of about 5.0 since minimum quantities of aluminum sulfate are absolutely necessary for good sizing.
U.S. Pat. No. 3,540,980 also discloses that the aluminum sulfate can be replaced entirely or in part by water soluble aluminate. In this case, aluminate solution is added to the aqueous fiber suspension to be sized until a pH of at least 9 is reached. Thereafter, the pH is lowered by means of an inorganic acid or a salt which undergoes an acid reaction, e.g. sulfuric acid or aluminum sulfate, to a pH no less than 4.5. After the addition of the resin sizing, the aluminum hydroxide produced during neutralization causes the aluminum resinate to flocculate and the resulting aluminum ions fix the resinate on the fiber. Under the stated conditions, the partial or complete substitution of aluminum sulfate by sodium aluminate leads to the desired increase in pH without a worsening of the sizing process being observed. Consequently, aluminum sulfate is no longer required to flocculate the sizing resin and to fix it to the cellulose fibers, it may be substituted by the addition of sodium aluminate and subsequent adjustment of the pH.
One drawback of the above process is now that it requires two stages and both stages must take place in highly diluted material suspensions; ancillary reactions and operational malfunctions are unavoidable if the process parameters are changed.